The well differentiated rat hepatomas have been the subject of extensive investigations in order to understand the molecular deviations that lead to malignancy. In the course of these investigations great biochemical variations have been noted to occur with the important common finding of the loss of regulation of fatty acid and cholesterol biosynthesis. Since acetyl CoA carboxylase catalyzes the rate limiting step in the biosynthetic sequence of reactions leading to fatty acids, its regulation has been investigated in hepatomas and was found to be aberrant. Personal and computer search (Medlar System) have revealed lack of information on the regulation of acetyl CoA carboxylase in mammary tumors although the enzyme has been purified from the normal glands. Availability of different transplantable mammary tumors with graded degree of differentiation including the estrogen dependent adenocarcinoma (DMBA 14) provide an excellent opportunity for comparative biochemical investigations on this enzyme. The following investigations are proposed: a) determination of the levels of acetyl CoA carboxylase in the normal lactating mammary glands of rats and in the transplantable mammary tumors to confirm and extend the observations made in hepatomas, b) to investigate the effect of hormones and cAMP on the rate of synthesis of this enzyme during mammary gland development and tumorgenesis, c) to purify the enzyme from normal glands and from at least two tumors for comparison of their properties and d) to explore the possibility of the presence of a modified form of the enzyme and its relationship to catalytic activity.